The present invention relates to a secondary air supply system for an internal combustion engine of the type wherein a secondary air is admitted via a one-way valve by utilizing a pressure change created by exhaust pulsation within an exhaust system of the engine.
Japanese Utility Model Publication No. 54-896 discloses a secondary air supply system which has a secondary air shut off valve disposed in a secondary air passage upstream of a one-way valve in order to promote reaction within a catalytic converter in an exhaust system at low engine temperatures and to prevent over heating of catalyst and burning thereof at high engine temperatures. The shut off valve is opened to allow the admission of secondary air via the one-way valve at low engine temperatures, whereas it is closed to suspend the secondary air admission at high engine temperatures.
This arrangement suffers from a drawback that when the shut off valve is closed, a portion of the exhaust gases enters into a casing of the one-way valve and the secondary air passage downstream thereof, staying there for a relatively long time while the engine operates at high temperatures, and water component of the exhaust gases is condensed and causes a rust on the one-way valve and the secondary air passage.
Japanese Utility Model Publication No. 57-17050 discloses a secondary air supply system wherein a small aperture is formed through a valve member or a valve seat of a secondary air shut off valve that is disposed upstream of a one-way valve. With the provision of the small aperture, a small amount of secondary air continues to flow through the secondary air passage in order to scavange or remove the exhaust gases even when the shut off valve is closed. However, since the small amount of secondary air is always supplied, over heating of catalyst and afterburning are likely to take place.